Stack injection molding is a process by which molding output can be increased by arranging mold cavities in a back-to-back manner in an injection molding apparatus having two or more parting lines. In order to deliver molding material to the mold cavities of a stack mold, the molding material is transferred from the molding machine across a parting line defined between a stationary side and a moving center section of the molding apparatus to a hot runner system that is disposed within the moving center section.
Various solutions have been proposed for transferring the molding material across the parting line of a stack mold to the hot runner system. For example, a sprue bar that travels with the moving center section as the mold is opened and closed during the injection cycle can be coupled to the hot runner system to extend across the parting line to interface with the machine nozzle; however when the mold is opened the sprue bar is disengaged from the molding machine nozzle and provisions must be made to prevent the molding machine nozzle and the sprue bar from ‘drooling’ molding material. Further in some instances, on opening of the molding apparatus, the sprue bar remains between the separated parting surfaces and becomes a potential obstacle to newly molded article that are being ejected from the molding apparatus.
Valve to valve molding material transfer systems are known that offer another solution for transferring the molding material across the parting line of a stack mold to the hot runner system in which a first valve transfer member is fixed relative to the stationary side of the molding apparatus and a second valve transfer member is fixed to the hot runner system in the moving center section. When the stack molding system is closed, the first and second transfer members are engaged and the valves in each transfer member can be positioned to allow melt to flow across the parting line from the molding machine nozzle through the first transfer member to the second transfer member and into the hot runner system. Conversely, when the stack molding apparatus is opened, the transfer members are separated from each other to allow for an obstacle free drop area for the newly molded articles that are being ejected from the molding apparatus and the valves can be positioned to prevent the flow of molding material. In such systems it is common for a molding material to build-up between the engaging surfaces of the transfer members.
As such, a need exists in the art to overcome some of the problems associated with known solutions for transferring melt across a parting line of a stack mold.